Deoxyribozymes are DNA oligonucleotides that catalyze a variety of reactions. Through in vitro selection, deoxyribozymes have been identified that are able to join (ligate) two RNA substrates together via 2',5' linkages, forming branched RNA products. This catalytic activity opens the door to new methods of RNA derivatization and RNA labeling. The objective of this proposal is to develop deoxyribozymes that covalently link a derivatized RNA substrate (a "tagging" RNA) to target RNAs via 2',5'-branched linkages. These labeling deoxyribozymes will be identified by in vitro selection, with the goal of identifying deoxyribozymes that have minimal sequence requirements and are tolerant of useful biophysical labels such as fluorescein and biotin. These labeling deoxyribozymes are envisioned as general tools that could be used to label model RNAs for in vitro studies by methods such as FRET. Other uses are also possible by changing the functionality of the tagging RNA. Ultimately, these deoxyribozymes will be tested as mRNA labeling agents in vivo. The ability to covalently attach probe-containing tags to RNAs of interest in a cellular setting will be of great use for studies of development and disease. [unreadable] [unreadable] [unreadable]